The present invention relates to a corrosion-resistance Titanium (hereafter abbreviated to Ti) alloy, and a member comprising this corrosion-resistance Ti alloy.
It is generally known that commercially pure Ti is superior to stainless steel and copper alloy in corrosion-resistance. In commercially pure Ti, however, corrosion is caused in non-oxidizing acid having a high temperature and a high concentration. Moreover, crevice corrosion is caused in a chloride solution having a high temperature and a high concentration. As methods for preventing such corrosion, there have been investigated a method of adding an alloying element, a method of adding an oxidizing agent in corrosion environment, a method of subjecting commercially pure Ti to surface treatment, and the like. Above all, the method of adding an alloying element is most reliable. Any platinum group element such as Pd or Ru is effective. This is because the hydrogen generating overvoltage of Pd or Ru is small so as to promote anodic polarization of Ti promptly. That is, the following oxidization reaction advances promptly on the surface of Ti, so that passivity oxidized film TiO2 is generated:
Ti+2H2Oxe2x86x92TiO2+4H++4exe2x88x92.
Thus, corrosion-resistance is improved. Specifically, Ti alloys such as Ti-0.15Pd alloy (ASTM Grade 7 and 11) have been developed. They are used in the fields of petroleum refining, petroleum chemistry plant and the like. However, the Ti-0.15Pd alloy has a problem that it contains a relatively large amount of expensive Pd so that material costs rise. In the case that the Ti alloy contains a large amount of Pd, Pd oxide, which is commonly called Pd black, is produced on the surface at the time of pickling. Thus, the pickling is blocked, so that the alloy needs to be passed through a pickling line many times. Thus, production costs also become high.
Recently, therefore, a Ti alloy has been developed to which an iron group element is added in such a small amount that does not damage workability in order to suppress addition of Pd or Ru, which leads to an increase in costs, as much as possible and compensate for deterioration in corrosion-resistance based thereon. For example, there are suggested Ti-0.05Pd-0.3Co (ASTM Grade 30 and 31; Japanese Published Examined Patent Application No. 6-89423) and Ti-0.5Ni-0.05Ru alloy (ASTM Grade 13, 14 and 15; Japanese Published Examined Patent Application No. 62-20269). These alloys are alloys wherein the added amount of an expensive platinum group element such as Pd or Ru is restricted to suppress an increase in costs and further deterioration in corrosion-resistance caused by a drop in the added amount of the platinum group element is supplemented by adding an additional element (Ni, Mo, Co or the like) contributing to an improvement in corrosion-resistance so far as workability is not largely damaged. However, it is unavoidable that workability drops by the addition of the corrosion-resistance improving element. Thus, it is difficult to use the above-mentioned Ti alloy for members which corrosion-resistance is required for and are subjected to a relatively strict cold work such as punch forming, for example, a plate type heat exchanger or components for an electrolysis bath in sodium electrolysis. Therefore, in spite of an increase in costs it is unavoidable to use, for purposes for which both corrosion-resistance and cold-workability are required, Ti-0.15Pd alloy (ASTM Grade 11), to which any corrosion-resistance improving element which damages workability, such as Ni, Mo or Co, is not added.
Incidentally, Ti is widely used in a condenser tube in a thermal power station and a nuclear power station, laying pipes in a high temperature and high-pressure chemical plant, such as a urea synthesizing plant, and a heat exchanger tube in a device for converting seawater to plain water. In accordance with uses, Ti may absorb hydrogen so that an accident may arise by hydrogen embrittlement. According to Japanese Published Examined Patent Application No. 4-57735, the Ti-0.15Pd alloy contains a large amount of Pd so that it has insufficient hydrogen adsorption resistance, as compared with commercially pure Ti. Thus, the publication suggests a Ti alloy wherein the amount of Pd is adjusted into the range of 0.03 to 0.1% [for example, Ti-0.05Pd alloy (ASTM Grade 16 and 17)]. In actual use, however, the hydrogen absorption of Ti is complicatedly related to surface states (roughness and finishing methods), grain size and use environment. Therefore, even if the Ti alloy wherein the amount of Pd is controlled to the range of 0.03 to 0.1%, hydrogen absorption is occasionally caused.
The inventors paid attention to the above-mentioned situations and have made the present invention. Thus, an object of the present invention is to provide a Ti alloy that is lower in price and better in corrosion-resistance than the Ti-0.15Pd alloy, which is widely used. Another object of the present invention is to provide a corrosion-resistance Ti alloy having a cold-workability (press formability) that is not less than that of the Ti-0.15Pd alloy, and a corrosion-resistance Ti alloy excellent in resistance against hydrogen absorption.
The corrosion-resistance Ti alloy according to the present invention, which attains the above-mentioned object, comprises Pd in an amount of 0.020-0.050 mass %, and comprises one or more platinum group elements other than Pd in an amount of one-third or more of the mass of Pd, and the balance being composed of allowable components and Ti.